Consumer packagings for drinks such as juice and milk are often manufactured from a flexible packaging material which, through cutting, bending and sealing, is formed into a filled and closed packaging container of the desired shape. The packaging material is usually a laminate which contains a bearer layer of fibrous material, e.g. paper, which is coated at least on one side, facing the contents, with a liquid-tight, thermoplastic material. The packaging laminate can also contain another layer of plastic or metal foil to ensure a better lightproof quality, gas barrier or tolerance of liquids. While the packaging material is still in strip or sheet form it is usually also provided with a pattern of bend lines or fold lines which weaken the material linearly and facilitate the forming of the same into packaging containers of the desired shape by bending. The bend or crease lines are obtained in the conventional manner, that is to say through the packaging material in strip form being passed between rollers with male and female tools, which press the desired pattern of crease lines into the material. The fold line pattern obtained thus exhibits a positive and a negative side, i.e. the linear deformation of the material caused by the fold tool results in raised fold lines on the one, positive side of the material and corresponding linear depressions on the opposite, negative side of the material. The weakening of the material obtained through the creasing process is caused by the fact that the fiber layer of the material is distorted so that the fibers in the region of the crease line are displaced in a wave-like manner, but are not broken off or cut off. No real reduction in the thickness of the material in the crease line takes place, but only a displacement of the material from the negative side to the positive side. Conventional crease lines thus give a linear weakening of the material, but the elasticity of the material is to a large extent retained, since no reduction in the material thickness in the crease lines and no cutting off of fibers takes place. The crease lines certainly achieve thereby a simpler and more precise process of bending, but the material retains its elasticity and, in the absence of external forces, it attempts to straighten out again to the original, mainly flat position. Conventional, folded crease lines do offer a sufficient weakening of the material for the majority of purposes, but when extra high demands are made for accurate shaping of the packaging or sharp, straight bends, a further weakening of the material is required, which not only facilitates the bending but also "kills" the elasticity of the material so that the material, to the greatest possible extent, remains in the folded position in the absence of external forces.
In packaging laminates which contain a central bearer layer of fibrous, liquid-absorbent material, it is a well known practice to ensure a folding over of the longitudinal edge of the strip of packaging material which, after being shaped into a packaging container, is coated on the package interior to prevent harmful contact with the contents in liquid form. The folding over of the edge of the strip of packaging material is done in several stages. First, a milling or grinding of the edge is carried out with the aim of reducing the material thickness so that the thickness of the edge after folding over is mainly the same as the total thickness of the material. Then the area of reduced thickness is provided with a longitudinal crease line mainly extending centrally in this area, after which the edge is doubled over and sealed within the area of reduced thickness. With certain types of material and material thicknesses it has proved difficult with conventional folding to ensure a sufficiently great weakening to make sure that the edge that is folded over remains in the doubled over position until the sealing has been completed. This has resulted in the width of the folded area being successively reduced so that finally the edge remains unfolded, which, if it is not detected, causes suction of the edge and leakage in the finished packaging container.
Bend lines which weaken the material to a greater extent than conventional crease lines can be achieved with the aid of a well known method, which is also used for reduction of thickness of larger material parts, e.g., in joints overlapping each other. In this method one side of the strip of packaging material is subjected to a milling or grinding process at the same time as the strip of material passes through a master tool, i.e. a tool which is provided with a pattern of the raised areas which serve as a holder in the milling and grinding process. For rational use several master tools must be applied on a counter roll and the master tools must be given individual shape for each type of weakening pattern that must be ground on the material, which is shown to be unreasonably expensive in manufacturing the packaging material for a large number of different types or sizes of packaging containers. In addition the method gives a line with worse definition, i.e. the transition from the ground area to the adjacent, unground area of the packaging material is gradual and not distinct.
A further manner of ensuring weakened or thinned areas of material is to use a conventional grinding or milling which results in a pattern of recessed grooves in one side of the material. These grooves serve as indications for folding, but they are very badly defined with sliding transfer to the unground material and therefore give badly defined, crooked crease lines. The method further entails a considerably greater removal of the fiber material, which produces great quantities of shavings that have to be handled.
There is thus a need within the packaging material industry to ensure a method which can provide at a reasonable cost a packaging material with well defined crease lines in a desired pattern.